Optimised cycling stability of sorption enhanced chemical looping steam reforming of acetic acid in a packed bed reactor

The cycling stability of reactor bed materials during the production of enhanced high purity hydrogen in the sorption enhanced chemical looping steam reforming (SE-CLSR) of acetic acid was studied and compared with the conventional steam reforming process. A packed bed reactor was used at 1 atm with a nickel catalyst supported on calcium aluminate intimately mixed with CaO in the role of high temperature CO2 sorbent. Twenty cycles of SE-CLSR were conducted under combined NiO-reduction/HAc steam reforming at 650°C, at feed molar steam to carbon ratio of 3 and WHSV of 1.18 hr-1, cyclically alternating with air feed to perform the coupled Ni-oxidation and CaCO3 calcination at 850°C. Sustained and consistent reforming was achieved in excess of 80 % of HAc conversion across all 20 SE-CLSR cycles; this was accompanied by hydrogen yield efficiencies exceeding 78% when compared to equilibrium values at same conditions. However, by the end of the 20th cycle, the extent of CaO carbonation had dropped to about 50% of that observed in the first cycle. Five SE-CLSR cycles were run using steam hydration at 250 °C prior to the fuel feed with the aim of improving sorbent conversion. A higher hydrogen yield was observed with an increase in fuel conversion. The sorbent conversion was also stable across all 5 SE-CLSR cycles when performed with sorbent pre-hydration using steam. This was attributed to an increased carbonation rate during the sorbent’s pre-breakthrough period. Enhanced auto-reduction of the NiO catalyst resulting from sorption of the CO2 product was also observed, and an improved sorbent conversion outlook over the cycles was investigated. TEM and SEM images indicated that carbon formed during the fuel-feed stage was eliminated during the cyclic oxidation step.